The serendipitous discovery of a group or cluster of young galaxies at z similar or equal to 2.40 in deep Hubble Space Telescope WFPC2 images

We report the serendipitous discovery of a group or cluster of young galaxies at z similar or equal to 2.40 in a 24 orbit Hubble Space Telescope/WFPC2 exposure of the field around the weak radio galaxy 53W002. Potential cluster members were identified on ground-based narrowband redshifted Ly alpha images and confirmed via spectroscopy. In addition to the known weak radio galaxy 53W002 at z = 2.390, two other objects were found to have excess narrowband Lya! emission at z similar or equal to 2.40. Both have been spectroscopically confirmed, and one clearly contains a weak active galactic nucleus (AGN). They are located within 1' of 53W002 or similar to 0.23 h(100)(-1) Mpc (q(0) = 0.5) at z similar or equal to 2.40, which is the physical scale of a group or small cluster of galaxies. Profile fitting of the WFPC2 images shows that the objects are very compact, with scale lengths similar or equal to 0''.1 (similar or equal to 0.39 h(100)(-1) kpc), and are rather faint (luminosities < L*), implying that they may be subgalactic-sized objects. We discuss these results in the context of galaxy and cluster evolution and the role that weak AGNs may play in the formation of young galaxies.